Voltage boosting detector circuit

ABSTRACT

Two cascaded capacitors, one in series and the other in shunt with a load, are connected across a source of amplitude-modulated oscillations with interposition of a small resistance, preferably the forward resistance of a series diode. The junction (A) of that resistance with the series capacitor is connected to the emitter of a transistor whose collector is energized with a high driving voltage (positive in the case of an NPN transistor) and whose base is tied to a tap on a voltage divider inserted between that collector and the opposite, usually grounded source terminal. The section of the voltage divider between the base and ground may consist of the forward resistance of a single diode, designed to balance the base/emitter threshold of the transistor whereby the aforementioned junction (A) is at ground potential when the source voltage peaks with a polarity opposite that of the collector potential. A biasing battery may be inserted between the voltage divider and ground to provide a voltage pedestal of the same polarity as the collector potential.

FIELD OF THE INVENTION

Our present invention relates to a detector circuit of thevoltage-booster type designed to translate the envelope of anamplitude-modulated carrier oscillation into an unbalanced signalvoltage whose magnitude with respect to ground or some other referencepotential substantially equals the peak-to-peak voltage differencebetween the positive and negative carrier halves.

BACKGROUND OF THE INVENTION

A conventional detector circuit of this type comprises two cascadedcapacitors, one of them in series and the other in shunt with a load,connected across the carrier source with interposition of a series diodebetween the first-mentioned capacitor and the load, the forwardresistance of this diode serving as a charging resistance for the shuntcapacitor. Another diode is connected between the usually groundedcommon input and output terminal of the circuit and a junction pointformed by the series capacitor with the series diode. One of thehalf-cycles of the carrier oscillation, i.e. the one (hereinafterassumed to be positive) which passes the series diode, charges the shuntcapacitor to a potential somewhat less than the (positive) voltage peak,the difference being due to the forward voltage drop of that diode. Uponthe next half-cycle, the junction point between the series capacitor andthe series diode is maintained by the shunt diode at a potentialsomewhat more negative than the reference potential (normally ground),owing to the forward voltage drop of the shunt diode. On thepositive-going swing of the carrier oscillation, the voltage V_(A) atthe junction point reaches a value which falls short of the peak-to-peakvoltage difference of the input oscillation by a value V_(D)corresponding to the aforementioned voltage drop.

OBJECTS OF THE INVENTION

The general object of our present invention, therefore, is to provide animproved voltage-boosting detector circuit of this character whicheliminates the voltage shortfall referred to.

Another object is to provide means for introducing a desired voltagepedestal, different from that of the common terminal, at theaforementioned junction point.

SUMMARY OF THE INVENTION

In a detector circuit according to our invention, the shunt diode of theconventional system is replaced by a transistor of the usual bipolartype (with collector, base and emitter electrodes) whose emitter is tiedto the junction point between the series capacitor and the chargingresistance for the shunt capacitor, that resistance being notnecessarily a diode. The collector of the transistor is connected to asupply of driving voltage (positive in the case of an NPN transistor)while its base is tied to a tap of a voltage divider inserted betweenthat supply and the common input and output terminal of the circuit. Thevoltage V_(A) of the emitter and therefore of the junction point, on ahalf-cycle (here assumed to be negative) of the input oscillationcausing the transistor to conduct, will then differ from the potentialof the voltage-divider tap by the base/emitter threshold voltage V_(be)of the transistor. The tap voltage can be so chosen that the extremevalue of the emitter voltage V_(A) equals the reference potential of thecommon terminal. Alternatively, voltage V_(A) may be made more positiveor negative (with an NPN or a PNP transistor) than that referencepotential with the aid of a suitable biasing source inserted between thevoltage divider and the common terminal.

BRIEF DESCRIPTION OF THE DRAWING

The above and other features of our invention will now be described indetail with reference to the accompanying drawing in which:

FIG. 1 is a circuit diagram of a conventional voltage-boosting detector;

FIG. 2 is a diagram similar to FIG. 1, illustrating our presentimprovement;

FIG. 3 is another diagram similar to FIG. 1, showing an alternateembodiment of our invention; and

FIG. 4 illustrates a partial modification of the circuits of FIGS. 2 and3.

SPECIFIC DESCRIPTION

The conventional detector circuit of FIG. 1 comprises a source ofamplitude-modulated carrier oscillations, shown as analternating-current generator G, connected between an input terminal 1and a grounded bus bar 3 serving as a common input and output terminal.A load L, which may be an operational amplifier, is connected across busbar 3 and an output terminal 2.

A series capacitor C' and a series diode D' lie between terminals 1 and2, forming with each other a junction point A which is returned toground through a shunt diode D". When the oscillation of generator Gdrives the input terminal 1 positive with reference to ground, capacitorC' transmits a positive-going pulse via diode D' to capacitor C" whichtherefore acquires a positive charge. On the subsequent negativehalf-cycle, this charge is trapped by the two cascaded diodes D' and D",except for a high-resistance leakage path through the load L or someother nonillustrated impedance defining with that capacitor a network ofdesired time constant. When input terminal 1 goes negative, junctionpoint A can follow that voltage swing until its potential falls belowground potential to a level -V_(D) determined by the forward drop V_(D)of diode D". On the next positive swing, point A starts at thispotential -V_(D) and eventually reaches a peak V_(p) +|V_(n) |-V_(D)where V_(p) and V_(n) are the positive and negative amplitude peaks ofthe input voltage from oscillator G. As the carrier amplitude changes,voltage V_(A) follows suit with the same difference V_(D).

In FIG. 2 we have illustrated our improved detector circuit wherein thejunction point A is tied to the emitter of an NPN transistor T whosebase is grounded through the shunt diode D". The collector of transistorT is connected to a supply of positive voltage +V_(c) and is alsoreturned to the base through a resistor R which forms a voltage dividerwith diode D". The potential of a tap B on that divider, to which thetransistor base is connected, is a small fraction of the collectorvoltage V_(c) inasmuch as the divider section lying between point B andground is constituted by the forward resistance of diode D". Thatforward resistance is substantially equal to the resistance of thebase/emitter path of transistor T in its saturated state whereby thevoltage drops across that path and across the diode D" balance eachother; the diode and the transistor may be easily so chosen that thethermal coefficients of these two resistances are substantiallyidentical so that the balance is not affected by changes in temperature.

During negative half-cycles of the voltage of generator G, transistor Tconducts and maintains the junction point A at a voltage V_(A)practically equal to ground potential. Thus, the shortfall V_(D)discussed with reference to FIG. 1 has been eliminated and point Areaches a potential of V_(p) +|V_(n) | on the next positive swing.

As shown in FIG. 3, the single diode D" of the voltage dividerillustrated in FIG. 2 may be replaced by a cascade of such diodes D₁, D₂. . . D_(n) whose combined resistance, of course, exceeds that of thebase/emitter path of transistor T though being preferably of the sameorder of magnitude. This insures that the minimum voltage of junctionpoint A is always positive with reference to ground, e.g. at +V_(D)whereby the voltage on terminal 2 will range between ground and V_(p)+|V_(n) |. The same effect can be had by inserting a source S of biasingvoltage, here shown as a battery, between the voltage divider and groundwith the proper polarity, i.e. with an ungrounded positive terminal inthe case of an NPN transistor. While the two features have been showncombined in FIG. 3, it will be apparent that either of them may beemployed separately.

In some instances the diode D' of FIG. 2 or 3 could be replaced by asmall resistor R' as illustrated in FIG. 4, especially with circuitsdesigned for high carrier frequencies in which the flow of a significantdischarge current from output terminal 2 back to junction pont A duringalternate half-cycles is nonobjectionable. Such resistors, substantiallysmaller than resistor R, may also be used in lieu of diodes D" or D₁ -D₄ in FIGS. 2 and 3.

Naturally, all the polarities will have to be reversed if a PNPtransistor is used in place of NPN transistor T.

Our improved detector shown in FIGS. 2 and 3 may be realized byintegrated circuitry.

We claim:
 1. A voltage-boosting detector circuit comprising:an inputterminal and a common terminal connectable across a source ofamplitude-modulated oscillations; an output terminal connectabletogether with said common terminal across a load; a series capacitorinserted between said input and output terminals; a shunt capacitorinserted between said output and common terminals in cascade with saidseries capacitor; a charging resistance for said shunt capacitorinserted between said series capacitor and said output terminal, saidcharging resistance and said series capacitor having a common junctionpoint; a transistor with a collector, an emitter and a base, said basebeing tied to said junction point; a supply of driving voltage for saidtransistor connected to said collector; and a voltage divider insertedbetween said supply and said common terminal, said base being connectedto a tap on said voltage divider.
 2. A detector circuit as defined inclaim 1 wherein a section of said voltage divider lying between said tapand said common terminal has a resistance on the order of magnitude ofthe base/emitter resistance of said transistor.
 3. A detector circuit asdefined in claim 2 wherein said section includes a generator of biasingvoltage with a polarity corresponding to that of said supply.
 4. Adetector circuit as defined in claim 2 wherein said section comprises adiode conductively inserted between said supply and said commonterminal.
 5. A detector circuit as defined in claim 4 wherein theforward resistance of said diode and the base/emitter resistance of saidtransistor have substantially the same magnitude and thermal coefficientof resistance.
 6. A detector circuit as defined in claim 2 wherein saidsection comprises a plurality of cascaded diodes.
 7. A detector circuitas defined in claim 1 wherein said voltage divider includes a series ofdiodes between said tap and said common terminal.
 8. A detector circuitas defined in claim 1 wherein said charging resistance comprises theforward resistance of a diode.